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Half plant, half predator, all weird.

I was planning on brushing up my knowledge of chloroplasts today, as next week I'm starting a plantsci course for my options lectures, but I got sidetracked by Captain Skellet alerting me to Hatena. I've heard of several organisms containing proto-plasmids; symbiotic chloroplasts which haven't completely been endosymbiosed, but Hatena was a new one so I went to look it up. And I'm very glad I did, because it's pretty amazing.

Hatena, taken from the reference. Green blob is the symbiont. Scale bar is 10um

Quick background: chloroplasts are little membrane enclosed vesicles in plants which carry out photosynthesis. Current theory for how they developed is that they were once free-living bacterial type organisms (cyanobacteria) which were engulfed by a larger cell and over time lost their own identity and became little photosynthesising factories inside the larger cell. (I've got another post on it here for anyone particularly interested in the subject.)

Hatena arenicola doesn't have a chloroplast, but it does have a symbiotic relationship with another organism; nephroselmis. The nephroselmis is always found in the same place in the Hatena, and carries out photosynthesis to provide energy for both of them. Unlike regular chloroplasts, nephroselmis has it's own proper nucleus and even it's own mitochondria although most of the internal cellular organisation and any kind of motile apparatus (such as flagella) has been lost.

The weirdest thing about these two organisms though, is their replication cycles. When Hatena replicates, the nephroselmis doesn't, and as a result only one of the offspring gets the photosynthesising symbiont. The other organisms remains colourless and develops a complex feeding apparatus at the apex of the cell, presumably as it can no longer rely on the symbiont for food. This wierd 'half plant, half predator' lifecycle is shown below. (Picture taken from the reference, scale bar 10um):

That's just weird. Seriously odd. The Hatena is able to move seemingly freely between being a predator consuming other cells for food, and being a plant-like organism, once it settles down with it's symbiotic partner. The grey non-symbiont organisms can be induced to take up free-moving nephroselmis and (in the words of the paper) "tentitavely" maintain a symbiotic relationship with them.

The paper suggests that Hatena cycles between these two modes of living, depending on circumstance. Thus the 'predator' grey cell shown above will continue eating fellow cells until it consumes a nephroselmis, at which point it degrades its complex feeding apparatus, accepts energy from the symbiont until it's ready to divide. One of the daughter cells will then go through the whole cycle again while the other remains as a non-predating plant. The authors freely admit that there is little evidence for much of these stages, but it seems a reasonable way to explain what is going on.

As this is clearly a very early stage in symbiotic capture it has important implications for the endosymbiotic theory of chloroplast evolution. Along with various other 'intermediate' symbionts (such as Karenia mikimotoi and Lepidodinium viride) the Hatena helps to show how chloroplasts might have first formed in the cellular ancestor of plants. Hatena and its symbiont have already acquired an intimate structural association, only the coordination of their cell cycles would be required to turn the nephroselmis into an internally replicating plastid.

Really cool. I like how this reinforces the credulity of the endosymbiont hypothesis! It would be great if it were somehow possible to artificially make the hatena and nephroselmis to become more dependent on each other, to see how far the endosymbiosis will carry...

Thanks for the paper summary, very neat. And thanks to Lynn Margulis and her colleagues who worked for decades (?) to get their symbiosis theory of the origin of chloroplasts accepted by a skeptical community.

Actually: do you happen to know anything about this weird creature's mating habits? I mean, when it comes to form gametes, does it keep its endosymbiont? Or is the gender of gametes perhaps influenced by the presence/absence of photosynthetic ability? I only asked these stupid questions because - once the organism looses the endosymbiont, that would certainly be a disadvantage - it would be really hard to encounter a new one somewhere out there (especially that this poor green alga is also dependent on its master...). So why don't they strive to keep it?

Bayndor: As this is only a single celled organism, it doesn't really form gametes. The diagram showing the splitting cell above is literally it's entire mating cycle. It would be highly beneficial for the both offspring of the hatena to keep a symbiont but if the symbiont isn't going to divide there's not a whole lot they can do about it!

As far as the symbiont is concerned that works out fine. It's still *in* the hatena after all, and it doesn't have to go through the complex and potentially mutagenic process of division in order to survive, just hang on for the ride while the cell around it splits in half.

Hi, thank you so much for writing this blog, this helps me a lot in my research.

Could you help me out in my research, I was wondering, why do you think Nephroselmis isn't digested by Hatena? And could you think of a mechanism on how Nephroselmis could survive being engulfed by Hatena and not be digested? Hope this isn't too much to ask.

Hi! Good luck with your research. I don't actually work with or study Hatena, but I can certainly try and answer your questions with my own opinions about the subject.

Why the Nephroselmis isn't digested: There are several bacteria that can evade the host digestion systems and survive inside macrophages that envelope them. Nephroselmis could have a very strong cell wall, or a modified cell wall to avoid the destructive enzymes sent out by the Hatena. It might also be able to signal to the Hatena not to digest it, or release a signal which supresses the digestive enzyme release.

In case you are interested, some common bacterial strategies for preventing digestion by the human macrophage cells was discussed by Michinaga Ogawaa and Chihiro Sasakawa in a paper titled "Bacterial evasion of the autophagic defense system" (you can easily find it by googling the title)

Hello there, thank you so much for writing this article. This topic is so fascinating! It also helps my research for my Cell Biology course at The Master's College in California. I know that we will never fully understand the complexity until we can ask God, the creator, face to face but have you come across any more information regarding Nephroselmis not being digested by the Hatena? ...Considering that this was written about three years ago. Thanks!